The present invention relates in general to polyethylene blends prepared so as to achieve a unique solid state processability, and more specifically to polyethylene blends formed of a major fraction of high density polyethylene (hereinafter referred to as HDPE) blended in solution with a minor fraction of low density polyethylene (referred to as LDPE) such as a branched low density polyethylene, a linear low density polyethylene or a low density polyethylene copolymer.
Enhancement of processability, improvement of product properties and/or reduction of cost for a polymer is often achieved by mixing into the polymer one or more different polymers to form a blend. There are basically two types of blends, compatible blends and incompatible blends. The polymer components in compatible blends are thermodynamically compatible with each other and mix more or less on molecular scale forming a single homogeneous phase. On the other hand, the polymer components in incompatible blends are thermodynamically incompatible and stay as their own separate phases of relatively large macroscopic sizes in the order of 10 microns or more. Polymer blends are usually prepared by mechanically mixing or blending the component polymers using a processing equipment such as extruder or banbury. Alternately, polymer blends may be prepared by dissolving and mixing the component polymers in a solvent. Mechanical mixing is preferred to solution mixing because of the difficulty of handling solvent in solution mixing. The structure, phase sizes and properties of a blend depends on the method and degree of mixing.
Yoshimura et al., U.S. Pat. No. 4,542,886 issued Sept. 24, 1985, discloses that the properties of polyolefine films are improved by mechanically mixing soft polyolefine resin such as low density polyethylene with rigid polyolefine resin such as polypropylene. Stearns et al, U.S. Pat. No. 4,107,243 issued Aug. 15, 1978, discloses a method for preparing thermoplastic fibril by discharging a two-phase mixture of polymer and solvent through a nozzle. The most relevant of these patents to the present invention is Salyer and North U.S. Pat. No. 4,115,499 issued Sept. 19, 1978, disclosing that high density polyethylene (HDPE) pellets mixed with low density polyethylene (LDPE) powders can be used as casting materials. However, the powders and pellets of the Salyer et al. patent are not fused together before usage, and in the casting operation, the HDPE pellets do not undergo any substantial amount of deformation and retain their own very large pellet size. The polyethylene blends are heated to a temperature above the melting point of the LDPE component but below or just above the melting point of the HDPE component in casting, such that the molten LDPE in the blends provides fluidity for processing.
Polymer processing usually involves transformation of a solid polymer into molten state, forcing the polymer melt into a desired shape and then solidifying the polymer melt in its desired shape. Such polymer processing is energy intensive because of the large heat content of polymeric materials and also quite slow because of the low thermal conductivity of polymeric materials. Solid state processing at a temperature below the melting point of polymer such as stamping or forging is desirable since it is energy efficient, fast and economical. This invention discloses polyethylene blends prepared in a particular manner to achieve unique solid state processability.